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How to Change a Furnace Filter (and How Often)

Changing your HVAC filter is the cheapest, highest-return maintenance task there is. Here's how to find the right filter, change it correctly, and how often to do it.

2 min read
In your maintenance planReplace HVAC air filterSee the cadence, priority, and steps for Heating & cooling (HVAC).

If you do only one maintenance task on your home, make it this one. Changing the HVAC filter costs a few dollars, takes two minutes, and protects a system that costs thousands to replace. A clogged filter is behind a huge share of avoidable HVAC breakdowns and high energy bills.

Why it matters so much

Your furnace or air handler pulls all your home's air through that filter. When it clogs:

  • Airflow drops, so the blower strains and uses more electricity.
  • The AC coil can freeze in summer or the furnace can overheat in winter.
  • Dust bypasses the filter and coats the condenser coils and blower, degrading efficiency.

A clean filter prevents all of that for the price of a coffee.

Step by step

  1. Find the filter. It sits in a slot in the return duct or in a door on the furnace/air handler — often a 1-inch slot near where a big duct meets the unit, sometimes behind a return-air grille on a wall or ceiling.
  2. Note the size and airflow arrow. The dimensions (e.g. 16x25x1) are printed on the frame. So is an arrow showing airflow direction — it always points toward the furnace/blower, away from the return duct.
  3. Turn the system off at the thermostat (optional, but it keeps dust from blowing while the slot is open).
  4. Slide the old filter out and check it against the light. If you can't see through it, it was overdue.
  5. Slide the new filter in with the arrow pointing the same way the old one did — toward the unit.
  6. Mark the date on the filter frame or set a reminder, and turn the system back on.

How often, really

  • 1-inch fiberglass or pleated: check monthly, replace every 1–3 months.
  • Pets or allergies: lean toward monthly.
  • 4–5 inch media filters: every 6–12 months.

A note on MERV ratings

Filters are rated by MERV (how fine they filter). Higher isn't always better: a very high-MERV filter in a system not designed for it can restrict airflow and strain the blower. MERV 8–11 is the sweet spot for most homes. When in doubt, match what the system came with.

Make it automatic

The filter is easy to forget — until the AC freezes on the hottest day of the year. Build your free Owner Tools and we'll remind you on the right schedule, alongside the annual HVAC tune-up and everything else your home needs.

Frequently asked questions

How often should I change my furnace filter?+
Every 1–3 months for standard 1-inch filters. Change monthly if you have pets, allergies, or run the system hard; every 3 months is fine for a low-traffic home with no pets. Thick 4–5 inch media filters can last 6–12 months. Check it monthly and replace when it looks gray and loaded.
What happens if you don't change your air filter?+
A clogged filter chokes airflow, making the blower work harder, raising energy bills, and reducing heating and cooling. In bad cases it causes the system to overheat or the AC coil to freeze, leading to expensive repairs. It also lets more dust into the air and onto the equipment.
How do I know what size filter I need?+
The size is printed on the edge of the old filter (like 16x25x1). If it's missing, measure the filter slot's length, width, and depth. Buy the nominal size that matches; never force a filter that doesn't fit, and don't run the system without one.

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